1. Field of the Invention
The present invention relates to heat pipes and similar devices for collecting, transferring and extracting thermal energy.
2. Prior Art
Heat pipes are known which include a heat conductive shell enclosing a quantity of heat transfer fluid. Such pipes are often essentially evacuated except for the heat transfer fluid. The space not occupied by heat transfer fluid in its liquid state can be referred to as the "vapor field". Heat pipes can include an internal wick substantially covering the inner periphery of the shell for transporting heat transfer liquid by capillary action to the heat absorption portion of the shell inner periphery. Such heat pipes will transfer thermal energy from a heat absorption or collection part of the outer surface of the heat pipe shell to a heat extraction part of such outer surface. Such a heat pipe is disclosed in U.S. Pat. No. 3,229,759, and is discussed in an article entitled "The Heat Pipe" by G. Yale Eastman in the Scientific American for May 1968, Vol. 218, No. 5 page 38.
U.S. Pat. No. 3,779,310 of George F. Russell for a "High Efficiency Heat transit System" discloses a heat pipe having heat extracting means enclosing one end of the heat pipe. The heat extracting means supplies a cooling fluid flowing around the outer surface of the enclosed end of the heat pipe to extract heat from the pipe outer surface and to promote condensation of heat transfer vapor within the pipe.
In use, one end of a heat pipe serves as a thermal energy collector and the other end serves as a thermal energy dissipator. For example, heat pipes have been used as cooling probes for electronic devices whereby one end of a heat pipe protrudes from an enclosed hotbox of semiconductors. Heat inside the box is absorbed by the inner end of the pipe and then transferred to and dissipated by the protruding end. This type of use results in a large longitudinal temperature differential. In the above example, if the outer end of a heat pipe six inches (15.24 cm) long protrudes into a 70.degree. F. (21.degree. C.) ambient atmosphere, the temperature could be 250.degree. F. (121.degree. C.) at the inner heat pipe end and only 75.degree. F. (24.degree. C.) or 80.degree. F. (27.degree. C.) at the outer heat pipe end.
It would be more efficient if an operating heat pipe were maintained at a uniformly low temperature throughout its length because the ability of a body to absorb thermal energy is a function of the difference in absolute temperature between the body and the medium from which the energy is transferred. The hotter the body is in relation to the transferring medium, the less energy it will absorb. Here the Stephen-Boltzman law pertaining to black body radiation holds. The potential capability of a black body for radiating heat to its surroundings may be measured at any point as an amount equal to the 4th power of the differences in absolute temperatures between the ambient atmosphere and the body's temperature. A solar collector may therefore receive less energy on those portions of its surface where the temperature is high than other areas where the temperature is lower.
Photovoltaic cells used for converting solar energy into electrical energy are well known in the art, being discussed, for example, in McGraw-Hill Encyclopedia of Science and Technology (1971), Volume 12, beginning at page 519 under the heading "Solar Battery."
In a photovoltaic cell, solar rays impinging on one cell surface effect a direct electrical current flow between such surface and another cell surface through connecting circuitry. It is known that the electrical output of a photovoltaic cell can be increased by concentrating the solar rays impinging on the exposed cell surface, such as by refraction through a lens or reflection by a parabolic mirror. However, it is also known that impinging highly concentrated or intensified solar energy onto the exposed cell surface heats the cell, and if the temperature of the cell increases to above about 60.degree. C., the electrical output of the cell decreases approximately linearly in relation to temperature increase.
Heat produced in photovoltaic cells as a result of their exposure to highly concentrated solar energy has been extracted and dissipated by projecting cell fins or heat sinks. In U.S. Pat. No. 3,990,914 a heat pipe has been used to extract heat from a photovoltaic cell but such heat has been extracted from the end of the heat pipe.